In order to build microstructures, a sacrificial layer first must be deposited on a substrate, and then a structural component or layer is deposited on the sacrificial layer. The sacrificial layer is then removed, leaving a substrate with a structural component attached to, but spaced from, the substrate. Two types of processes are currently used to remove the sacrificial layer. One type is the wet-release method, and the other type is the dry-release method. However, problems exist with each method.
In a typical wet-release method an etching solution or etchant is applied between the substrate and the structural layer to remove the sacrificial layer. Then, the sacrificial layer is rinsed away by a rinsing solution. As the etching and rinsing solutions evaporate from the small space between the structural layer and the substrate, strong capillary forces between the two are created. As the volume of liquid trapped beneath the structural layer decreases by evaporation, the capillary forces become stronger. As a result of these forces, the structural layer begins to deflect toward the surface of the substrate. If the capillary forces are strong enough, and the structural layer weak enough, the structural layer deforms to the point where it contacts the substrate. At that point, intersolid forces are the strongest, and the structural layer may be permanently pinned to the substrate. Therefore, a need exists for a method that prevents the destructive effects of the capillary forces associated with wet etching techniques.
To overcome the destructive effects of the capillary forces, dry-release methods have been developed. While the dry-release methods for removing sacrificial layers are not troubled by capillary forces, they have their own distinct drawbacks. For example, the dry-release method of Saiki in U.S. Pat. No. 3,846,166, requires deposition of a structural layer on a sacrificial resin layer. However, because the resin layer melts or degrades at low temperatures (e.g., 300.degree.-400.degree. C.) many desirable materials for the structural layer which require higher deposition temperatures cannot be deposited upon the fragile sacrificial resin layer. For example, polysilicon, a preferred material for microstructures, requires a temperature of about 600.degree. C. for deposition.
Similarly, Bly in U.S. Pat. No. 4,849,070, teaches a dry-release method for removing a sacrificial layer in which the structural layer is built on top of a solid layer that can later be sublimed to free the structural layer. However, the choice of structural layer materials is again limited to lower temperature materials, as higher temperature materials cause the sublimable layer to disappear before deposition thereon. In addition, Bly teaches the production of permanent posts which support the structural layer above the substrate, and form part of the finished microstructure. However, the need exists in many applications for microstructures without such permanent posts.
Other dry-release methods known in the art include liquid freezing and sublimination methods and a photoresist refill and plasma-etching method. These methods also their own drawbacks and limitations under certain circumstances. Liquid freezing and sublimination methods may be unreliable and difficult to control. The freezing of the liquid between the structural layer and the substrate can result in an increase in volume which can fracture the microstructure. The photoresist refill and plasma-etching method is difficult to perform, primarily because time consuming and costly mixing of the solution are required.
Guckel et al. in U.S. Pat. No. 4,744,863 teaches the use of a piezoresistive sensor in conjunction with a cavity structure formed by depositing a polycrystalline silicon layer over a large silicon dioxide post or latice having smaller ridges leading outwardly from the central area to the lateral edges. The polysilicon layer is masked and etched to expose the lateral edges of the ridges, and then the entire structure is immersed in an etchant which will remove the ridges and the post but not the substrate. The cavity is then sealed by vapor deposition of polysilicon or silicon nitride. At column the concept of using capacitive deformation sensing is mentioned as an alternative, but is not taught.